Arteriovenous O2 content (a-vCO2) differences increase during exercise in normal subjects through several mechanisms including PO2, O2 pressure at which hemoglobin (Hb) is half saturated with O2 (P50), and Hb concentration changes. The present study was undertaken to evaluate how much these biochemical changes are relevant to a-vCO2 difference through exercise in patients with heart failure. Twenty-seven patients with congestive heart failure [10 patients in functional class A (peak exercise O2 uptake >20 ml x kg-1 x min-1), 9 in class B (20-15 ml x kg-1 x min-1), and 8 in class C (15-10 ml x kg-1 x min-1)] underwent a cardiopulmonary exercise test with once-per-minute simultaneous blood sampling from the pulmonary and systemic arteries for determination of Hb, PO2, PCO2, pH, O2 content (CO2), Hb saturation and lactic acid (pulmonary artery only), and calculation of P50. Analysis of data was done at six exercise stages: the first at rest, the last at peak exercise, and the second to the fifth at one-, two-, three-, and four-fifths of O2 consumption increase. a-vCO2 difference at peak exercise was 14.3 +/- 2.1, 16.9 +/- 2.4, and 14.7 +/- 2.1 (SD) ml/dl in class A, B, and C patients, respectively. The contribution of Hb, P50, and PO2 changes to the increments of a-vCO2 difference during exercise was 21, 17, and 63%, respectively; the only interclass difference observed was for P50, which plays a greater role in a-vCO2 difference in class A. Hb changes act mainly at the arterial site, whereas P50 and PO2 act at the venous site. Hb increase was constant through the test, venous P50 increase was greater above anaerobic threshold, and venous PO2 reduction was most remarkable at the onset of exercise; in class C patients, no venous PO2 change was recorded in the second half of exercise. Thus a-vCO2 difference increase during exercise is notable in patients with heart failure but unrelated to the severity of the syndrome. Hb, P50, and, to the greatest degree, PO2 changes participate in the increment of a-vCO2 difference. In class C patients, the lack of PO2 reduction in the second half of exercise suggests the achievement of a "whole body critical venous PO2."